In one, about 300 people watched, listened to or read arguments about war, abortion or music (country or rap – genres people tend to have strong feelings about).

Afterward, the volunteers were asked to judge the person who communicated the argument. Those who were exposed to someone they disagreed with tended to “dehumanise” the communicator. That is, they regarded the person as “having a diminished capacity to either think or feel”.

However, those who listened to the argument, either in a video or audio file, were less dismissive than those who read a transcript of the opposing opinion.

Beliefs that are communicated by voice make the communicator seem more reasonable, even human, according to Schroeder, an assistant professor at Berkeley’s Haas School of Business.

But those same beliefs are stripped of the humanising elements when the opinions are communicated on a piece of paper.

The research that forms the basis of the study began with a newspaper article, she said.

“One of us read a speech excerpt that was printed in a newspaper from a politician with whom he strongly disagreed,” Schroeder said in an email to The Washington Post.

“The next week, he heard the exact same speech clip playing on a radio station. He was shocked by how different his reaction was toward the politician when he read the excerpt compared to when he heard it. When he read the statement, the politician seemed idiotic, but when he heard it spoken, the politician actually sounded reasonable.”

In another experiment conducted by Schroeder and her colleagues, eight communicators discussed their support of one of the two main candidates in the 2016 presidential election. About 600 subjects rated the communicators by listening to or reading their opinions via video, audio, transcript or written text.

As in the first experiment, the volunteers devalued people who communicated an opinion they disagreed with. But once again, those assessments were softened when the opposing opinions were seen or heard rather than read.

“When two people hold different beliefs, there is a tendency not only to recognise a difference of opinion but also to denigrate the mind of one’s opposition,” the study’s authors wrote.

“Because another person’s mind cannot be experienced directly, its quality must be inferred from indirect cues.”

Those cues, involving the many characteristics of the human voice, are absent in written communication.

In an earlier study, Schroeder set up an experiment involving hypothetical employers and recruiters who watched, listened to or read pitches from job applicants.

As in the subsequent experiments, the employers and recruiters “rated a candidate as more competent, thoughtful, and intelligent when they heard a pitch rather than read it.”

The result was that those favorable impressions led the hypothetical employers to say they were more interested in hiring the job applicant.

Schroeder thinks her research can help explain – and maybe alleviate – the rise in polarising political opinions.

“In some ways, technology is making more of our interactions text-based,” Schroeder said.

“Many people receive the majority of their news from social media​ now. This can be dehumanising, and may increase polarisation. It’s easy to imagine how this could become cyclical; dehumanisation leading to more polarisation leading to more dehumanisation.”